Process for preparing bonded,nonwoven fabrics



United States Patent 3,420,724 PROCESS FOR PREPARING BONDED, NONWOVEN FABRICS Robert H. Saunders, 'Chadds Ford, Pa., assignor to Hercules Incorporated, a corporation of Delaware No Drawing. Filed May 18, 1965, Ser. No. 456,812 US. Cl. 156272 7 Claims Int. Cl. D04h 1/54 This invention relates to novel nonwoven fabrics and to a process for the preparation thereof.

Nonwoven fabrics are textile fabrics which are neither Woven, spun, nor made by conventional wool felting processes. Rather, they consist of an agglomeration of staple textile fibers interlocked to form a mat-like structure. It has been estimated that the market for such fabrics is currently in excess of 100 million pounds per year including both natural and synthetic textile materials. Typical applications where these fabrics find utility include filter cloths, clothing insulation, carpet backing, gasket material, blankets, doilies, dish cloths, surgical dressings, pennants, inner-soles for shoes and many others.

Both a wet and a dry process are known for formation of these fabrics. In the wet process, the fibers are slurried in water or a similar inert liquid, the slurry is spread uniformly on a fiat surface, the inert liquid is drained off and the web is dried under pressure to form the loosely agglomerated web of randomly arranged fibers. In the dry process, dry fibers are laid on a solid flat surface, as a conveyor, by mechanical means such as, e.g., a carding machine. The dry process can be used to lay down the fibers in either a random or oriented arrangement. A thorough discussion of the formation of nonwoven fabrics is presented in Nonwoven Fabrics by F. M. Buresh-Reinhold Publishing Co., New York, N.Y., 1962.

Regardless of the method employed to form the nonwoven structure, it is, at this point, a flimsy structure having virtually no tensile strength and unable to remain in one piece without support. To impart the desired tensile strength and cohesion to the structure, it is necessary that steps be taken to bond and interlock the fibers.

The most usual, and most economical means of accomplishing the bonding of the fibers is by means of a chemical bonding agent. In the past, these bonding agents have been employed in the form of emulsions, powder, solvents, or solutions applied to the agglomerated fibers and subsequently cured or otherwise caused to adhere thereto. While these prior art bonding agents have generally performed satisfactorily, they have been subject to certain objections. For example, the addition of the bonding agents has always involved several time consuming and troublesome handling operations. If the bonding agent is applied as a solution, or emulsion, not only is it necessary to spray or dip the fabric to contact it with the binder, but the liquid phase of the solution or emulsion must be removed. It is usually then necessary to heat or otherwise activate the bonding agent to cure it and adhere it to the fibers.

In accordance with this invention, bonded, nonwoven fabricsc'ontaining synthetic thermoplastic fibers are prepared by a process which avoids the above difficulties. This process comprises preparing a web containing thermoplas- 3,420,724 Patented Jan. 7, 1969 "ice tic synthetic organic, staple fibers, providing said web with intermittent dark and light colored areas, exposing the web to radiant heat energy whereby the thermoplastic material in the dark areas melts preferentially, and thereafter cooling the web so that the molten areas solidify and become bonded to the fibers in the light colored areas. The fabric can comprise exclusively thermoplastic fibers or it can be a blend of synthetic and natural fibers.

The synthetic, thermoplastic fibers employed in the nonwoven fabrics of this invention are prepared by the well known procedures normally used for the preparation of synthetic fibers. Briefly, this comprises extruding molten polymer through an orifice containing a plurality of very fine holes, and drawing the polymer away from the orifice at a greater rate than the rate of extrusion to effect a substantial draw down of the filaments in the molten state prior to solidification thereof. The solidified filaments are twisted into a multifilament yarn and normally are subjected to a cold draw at a temperature below the polymer melting point whereby a desired amount of molecular orientation is imparted to the polymer. The molecularly oriented filaments are then crimped, usually by mechanical means such as a stuffing box crimper. The crimped fiber is then chopped into relatively short staple fibers, usually on the order of one to several inches in length. Substantially the same operations can be employed with all synthetic fiber-forming polymers, with operating conditions varying to suit the polymer being processed.

While the procedure described is the procedure usually employed in the preparation of synthetic fibers, and the fibers used in the invention will normally be prepared by this technique, it must be understood that the process is not limited to such fibers. For example, if desired, the molecular orientation step may be omitted if the greater dimensional stability of the unoriented fiber under the influence of heat is desired. Uncrimped fibers likewise can be used if this is desirable.

The invention can be practiced with nonwoven webs prepared by any method known to the art. Thus, it is applicable to randomly deposited webs as well -as to webs having individual fibers oriented or aligned along a particular axis. The preferred webs are those wherein the fibers are randomly deposited since in these webs, the entanglement of the fibers yields a greater number of potential bonding sites.

The intermittent dark and light colored areas are readily provided by blending dark and light colored fibers during preparation of the web. Alternatively they can be provided by forming the web exclusively of light colored, e.g. natural color fibers, and randomly printing the same with dark spots. These are the preferred embodiments of the invention. Either technique will prepare a web having dark colored areas randomly distributed, not only on the surface but throughout the body of the web so that the finished fabric will be bonded at a large number of points throughout its thickness. In another embodiment of the invention, a master sheet containing alternating dark and light areas can be laid over the nonwoven web and radiant energy can be applied thereto whereby the overlay sheet heats preferentially in the dark areas, transmits this heat to the web, and melts it in selected areas. This embodiment is useful for formation of a web of a single color, which has aesthetic value in some instances.

In order to have the correct degree of bonding between fibers, it is preferable to have about 5 to 50% dark colored areas in the fabric. The correct degree of bonding is that point where the fabric has the necessary tensile strength combined with the desired degree of flexibility and hand. An insufiicient amount of bonding results in low tensile strength due to insufiicient adhesion between fibers. Such a fabric is easily destroyed by the stresses to which it is subjected in use. Too much bonding, on the other hand, results in a stiff fabric, too nearly resembling a fused fabric, lacking the pleasant feeling or or hand usually found in nonwoven fabrics. Such a fabric finds limited utility in most applications, since a pleasant hand is usually desirable.

The process of the invention is applicable to the formation of nonwoven fabrics containing any synthetic fiberforming thermoplastic polymer. Examples of such materials include polyolefins such as polyethylene, polypropylene, and poly(4 methylpentene-l); polyamides such as nylon; poly(ethylene terephthalate); acrylic polymers; vinyl polymers such as poly(vinyl chloride), poly (vinylidene chloride), poly(vinyl acetate) and copolymers of these materials, inter alia.

The process is particularly useful in forming nonwoven fabrics from polyolefin fibers. These fibers are difiicult to bind by the usual chemical bonding means since they normally adhere to only a limited number of other materials. Thus the choice of bonding agents is relatively limited for the practitioner who must work with polyolefins. With the process of the invention, the practitioner need not be concerned with selecting a bonding agent, since none is required.

A similar advantage is presented in the case of other synthetic fibers, though the problem of finding suitable bonding agents is not so acute with others as it is with polyolefins. In addition, in using the process of the invention, the practitioner who may wish to work with a variety of fibers is spared the necessity of stocking bonding agents specific to each type of fiber.

In the practice of the prior art procedures, relatively large quantities of the bonding agent are required in order to effect the optimum degree of bonding between fibers. In some cases this quantity is as high as and even 50% of the total fabric. This large amount of binder contributes undesirably to the total weight of the finished fabric in relation to the amount of fiber therein. The fabrics of the instant invention are composed substantially entirely of fibrous material and thus a fabric containing a given amount of fiber per unit volume can have strength equivalent to that of a chemically bonded material, but will be substantially lighter. Alternatively, a given weight of fabric can have a higher fiber content and exhibit equivalent strength.

The elimination of the chemical bonding of the fibers also leads to significant advantages in laundering and dry cleaning. In the known nonwoven fabrics and processes, it is not customary for the fiber and the binder to be of a similar material. Normally the binder is applied from an emulsion or solution and comprises a thermally curing or thermosetting material. Thus in many cases, the synthetic fiber portion of the fabric and the binder have distinctly different chemical reactivity and solubility characteristics. This frequently leads to a problem in selecting laundering and dry cleaning reagents since such reagents must be compatible with both fiber and binder. This problem need not be encountered with the fabrics of this invention since the entire fabric can be composed of the same material so that both binder and fiber have the same solubility and chemical reactivity. Thus any cleaning reagent known to be useful for the fiber can be employed without concern for its possible effect on the binder.

As previously stated, most nonwoven fabrics are characterized by a random arrangement of the fibers and whileit is possible to orient the fibers mechanically when forming the web, this is not normally done since the properties of such a web are not sufficiently improved over those of a random web to justify the increased expense of the required equipment. Theoretically, fabrics having their fibers oriented and aligned with respect to each other would be expected to exhibit greater tensile strength due to the distribution of the applied force along a greater number of fibers than is the case with randomly arranged fibers. However, this has not always been found to be the case with prior art nonwoven fabrics, apparently due to the fact that the strength gained from uniform distrfbution of the fibers is canceled out by the loss of strength due to absence of entanglement of fibers with one another. Thus the prior art nonwovens with fibers aligned along the axis of the fabric are usually no stronger, and frequently are even weaker than those whose fibers are randomly arranged.

The fabrics produced by the preferred embodiments of the instant invention, on the other hand, represent a distinct improvement over the prior art in this respect. Since the individual fibers employed in preparing these fabrics are molecularly oriented, the dark colored fibers undergo a considerable shrinkage upon their melting and consequent disorientation. This shrinkage causes the dark areas to form into small spheres with the randomly arranged fibers bonded within the spheres. As the spheres shrink, the fibers contacted by each sphere are pulled into a relatively regular arrangement or alignment radiating from that sphere. As a result, the fabrics acquire some of the tensile strength characteristics which should theoretically be possessed by fabrics having their individual fibers aligned along a given axis, since the more regular alignment of the fibers allows a tensile force to be distributed along a larger number of fibers. In addition, since these fibers were originally laid down in a random arrangement, they also retain a great degree of desirable entanglement of the fibers resulting therefrom. The result is a fabric whose tensile strength is greater than that of either an oriented or randomly arranged fabric prepared according to prior art methods.

The examples in the following table are presented to illustrate the operation of the process of the invention and to demonstrate the desirable properties of the novel fabrics produced thereby. The webs for all of the fabrics were prepared by a standard carding operation, the fibers being laid randomly upon a flat backing. Some of the webs were prepared with a mixture of natural colored fibers and fibers pigmented with carbon black, others were prepared from exclusively natural colored fibers and were printed with black ink containing carbon black pigment after being formed. The fabric was then compressed under a transparent cover to which the molten polymer would not readily adhere and was passed slowly under a bank of infrared bulbs at a distance of about 16 inches. Approximately 30 seconds exposure was required to melt the black fiber. Upon cooling, the black fibers solidified to bond the fabric.

The bonded fabrics were conditioned for 2 hours at 65% RH and 70 F. for two hours and then tested according to ASTM method D-1l17-59 to determine their tensile strength, elongation and fabric weight. In determining the tensile strength, the cut strip method (ASTM D-l1l7-59 Section 6) was used. Test specimens were 1 x 6 inch strips, which were tested in an Instron Tensile Tester with a 3-inch draw span, cross head speed set at 12 inches per minute and chart speed at 10 inches per minute.

Several control tests are included showing results of chemically bonding similar synthetic fibers 'by known prior art methods.

The data in the table clearly show the high tensile strength of the nonwoven fabrics of the invention as compared to prior art nonwoven fabrics of comparable sample weight.

Example Type Percent Method of Sample wt. Elongation, Tensile,

No. fiber black black oz./sq. yd. percent (lbs./in./oz./sq. yd.) Remarks lncorp.

Control A..- Polypropylene 2.6 1.50 Bonddeid with 30% Poly(vinyl chlor1 e 2. 7 1.0 Boznlgd with 30% Poly(vinyl ace- The dark colored areas in the nonwoven fabric can be of any color which exhibits a preferential absorption of heat as compared to the light colored fiber with which it is blended. The minimum difference in shade required is that which will permit the dark area to absorb sufiicient heat to melt while the light area remains cool enough that it is neither melted nor softened to a point where its molecular orientation is disturbed. Since synthetic textile fibers are normally white or approximately white in their natural state, it is preferred to employ natural colored fibers for the light areas, in which case almost any dark color fiber exhibits some efiicacy in the process. The preferred dark color fiber is black or brown since these colors exhibit the greatest preferential heat absorption. The heat required for melting will be absorbed by black or brown fibers well before a corresponding amount of heat is absorbed by the natural colored fibers. Accordingly, the danger of heating the natural colored fibers to a point where their molecular orientation is disturbed is at a minimum where black or brown fibers are used as the heat absorbing fibers.

However, it is also possible to use other dark color fibers such as, e.g., dark red, dark blue, or dark green if conditions of heating are so controlled that the light colored fibers are not heated sufliciently to melt them or otherwise disturb their molecular orientation. Pleasing decorative effects can often be obtained by employing dark colors other than brown or black and this is frequently desirable. In this connection, it is also possible to employ mixtures of colors.

As another alternative embodiment of the invention a Web can be formed of exclusively light colored fibers and it can thereafter be sprinkled randomly or in selected areas with dark colored granules of the same or a similar polymer, so that, upon application of heat, the granules melt preferentially to provide bonding to the fibrous polymer. In the case of some of the higher melting polymers, it is sometimes desirable to use this alternative method, employing as the granular dark colored polymer a lower melting material whereby the fusion step can be accomplished at a lower temperature and more quickly than if the bonding material is the same as the fibrous material. This helps protect the fibrous material from being overheated and perhaps having its orientation disturbed. This method is particularly useful in cases where the shade contrast between the dark and light areas is not great. When employing this embodiment, however, it is desirable to use polymers which have very similar properties-cg, polyethylene with polypropylene or a lower melting polyamide with a higher melting polyamide. Thus the similarity of properties between bonding agent and fibrous polymer referred to earlier is maintained.

What I claim and desire to protect by Letters Patent is:

1. A process for preparing a bonded, nonwoven fabric which comprises preparing a web comprising synthetic, organic, thermoplastic staple fibers, said web having intermittent dark and light colored areas, exposing the web to radiant heat, whereby the fibers in the dark areas melt preferentially, and thereafter cooling the web so that the molten thermoplastic fibers solidify and become bonded to the light colored fibers.

2. The process of claim 1 wherein 5 to 50% of the total fibers in the fabric are within the dark area.

3. The process of claim 1 where the synthetic, organic staple fibers are molecularly oriented.

4. The process of claim 3 where the synthetic, organic staple fibers are crimped.

5. A process for preparing a bonded nonwoven fabric which comprises preparing a web of randomly deposited molecularly oriented, crimped synthetic organic staple fibers, said web having intermittent dark and light colored areas with 5 to 50% of the total fibers in the fabric within the said dark areas, exposing the web to radiant heat whereby the fibers in the dark areas melt preferentially, and thereafter cooling the web so that the molten fibers solidify and become bonded to the light colored areas.

6. The process of claim 5 where the light colored fibers are their natural color.

7. The process of claim 5 where the thermoplastic polymer is polypropylene.

References Cited UNITED STATES PATENTS 2,774,129 12/1956 Secrist 161169 2,959,509 10/1955 Marshall 161-81 ROBERT F. BURNETT, Primary Examiner.

R. L. MAY, Assistant Examiner.

US. Cl. X.R. 

1. A PROCESS FOR PREPARING A BONDED, NONWOVEN FABRIC WHICH COMPRISES PREPARING A WEB COMPRISING SYNTHETIC, ORGANIC, THERMOPLASTIC STAPLE FIBERS, SAID WEB HAVING INTERMITTENT DARK AND LIGHT COLORED AREAS, EXPOSING THE WEB TO RADIANT HEAT, WHEREBY THE FIBERS IN THE DARK AREAS MELT PREFERENTIALLY, AND THEREAFTER COOLING THE WEB SO THAT THE MOLTEN THERMOPLASTIC FIBERS SOLIDIFY AND BECOME BONDED TO THE LIGHT COLORED FIGERS. 